This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Primary migration of petroleum compounds may be defined as the release of petroleum compounds from kerogen and their transport within and through narrow pores of a fine-grain source rock. Kerogen is solid, carbonaceous material found in sedimentary rocks. When kerogen comprises around ten weight percent or greater of the rock, the mixture is referred to as oil shale. This is true whether or not the mineral is, in fact, technically shale, that is, a rock formed from compacted clay. Kerogens, and the sediments that contain them, can comprise what is known as hydrocarbon source rock. Kerogen is chemically altered upon exposure to heat over a period of time. Upon heating, kerogen molecularly decomposes to produce oil, gas, and carbonaceous coke. Small amounts of water also may be generated. The oil, gas and water fluids are mobile within the rock matrix, while the carbonaceous coke remains essentially immobile.
Petroleum expulsion from their source rocks is the initial step in the migration process, during which the composition of the expelled petroleum is enriched in saturated and aromatic hydrocarbons while the retained bitumen is enriched in asphaltene and polar compounds. Numerous physical and chemical models have been proposed to explain petroleum expulsion and chemical fractionation; and, until recently, were largely empirical. The uncertainty in the fundamental principles and geochemical constraints of these processes contrasts with the considerable advances made in the understanding of source rock deposition, kerogen compositions, kinetics and mechanisms of petroleum generation and reservoir alteration processes.
Many expulsion models target the chemical or physical processes of oil moving within the source rock mineral matrix as the rate-determining step. Some considered the amount and type of organic matter as being critical to generating sufficient bitumen to exceed a saturation threshold. The establishment of effective and continuous migration pathways within the source rocks may be considered to be critical. Other models have considered pressure build-up from generation and compaction and the failure of the rock fabric forming micro-fracturing as a key element in expulsion. Still others have evoked gas availability and movement of oil in a gas or supercritical phase or movement of oil in an aqueous phase. These elements are controlled mostly by the sedimentary conditions during source rock deposition and by secondary diagenetic processes that occur during the evolution of sedimentary basins; consequently, the mechanisms that define oil movement will differ according to the lithofacies of the source rock.
A competing theory is that the rate-limiting factor for expulsion is the release of petroleum from its source kerogen. This hypothesis places little importance on movement of petroleum within the mineral matrix; rather, it postulates that the expulsion is controlled by adsorption of generated petroleum onto the surface of the kerogen and/or the absorption or diffusion of the hydrocarbons through the kerogen matrix. The concept that kerogen has an absorptive capacity to retain petroleum and only releases hydrocarbon-rich fluids once this capacity is exceeded may facilitate modeling efforts because it requires only knowledge of the kerogen and its petroleum products during basin evolution.
There is considerable evidence that expulsion is governed by the release of petroleum from kerogen. The most direct confirmation is the observation that the amount of extractable petroleum from kerogen isolates is comparable to that extracted from powdered rocks. Other empirical observations supporting this concept include linear correlations between Rock-Eval hydrogen index (HI) and expulsion efficiency and between Rock-Eval S1 and total organic content or TOC that are independent of thermal maturation. Conceptually, differences in generative yield and retention capacity could explain the apparently large differences in expulsion efficiencies between very organic-rich source rocks such as coals and oil shales. Previous efforts to model kerogen retention capacity are largely empirical. A relatively simple rule has been proposed that expulsion occurs when the amount of generated petroleum exceeds 200 mg/g C (+1 mg/g C for the pore space). This approach has been extended to individual hydrocarbon fractions to provide an empirical model of chemical fractionation.
A comprehensive theory of the fundamental principles of the expulsion process is slowly evolving. Early studies explored the concept that bitumen diffuses through the kerogen matrix and molecular diffusion was proposed as a mechanism for expulsion. However, it has been shown than the diffusion effects would preferentially expel fluids with the opposite compositional fractionation as that seen in nature (in other words, aromatics is greater than naphthenes which is greater than alkanes). It has been proposed that kerogen-fluid phase partitioning is more important that diffusivity. An additional proposal is that the compositional fractionation observed in expulsion was consistent with documented interactions between solvents and kerogen. Absorption processes, therefore, may be considered to be an important factor in determining the magnitude and composition of expelled petroleum. While surface adsorption may play some role, solvent-swelling experiments have shown that all types of kerogen have sufficient absorptive properties to explain residual bitumen concentrations in petroleum source rocks and coals. These swelling experiments demonstrated that kerogens and coals behave in manners similar to cross-linked polymer network.
The application of solution theory has been applied to model chemical fractionation during expulsion. In one such application of solution theory, several simplifying assumptions based on limited data have been made. Foremost is the simplification that the kerogen swelling ratio, Qv, exhibits a Gaussian distribution as a function the solvent solubility parameter, δ, with the peak maximum corresponding to the δ of the kerogen. From this, expulsion efficiency (EEF), defined as proportion of expelled oil to retained bitumen, has been modeled as a function of kerogen generative potential and maximum volumetric swelling ratio, Qv. Using a fixed Qv value of 1.6 for kerogen, EEFs of 0.9 and 0.7 for a hydrogen-rich and a hydrogen-lean kerogen (HI=538 and 215 mg petroleum/g TOC, respectively) were selected. With the amount of retained and expelled products defined, compositions were calculated for methane and lumped petroleum fractions by comparing their solubility parameters with that of kerogen (δ=19.4 (J/cm3)1/2).
Based on this, it has been concluded that the Hildebrand solution theory predicts the chemical direction, but not the extent of the chemical fractionation observed between natural retained bitumen and expelled oil. In particular, one implementation of the theory predicts that preferential expulsion occurs where saturated hydrocarbons>aromatic hydrocarbons>polar compounds, but the modeled compositions of expelled oil are depleted in saturated hydrocarbons (>30%) and enriched in aromatic hydrocarbons and polar compounds relative to reservoir fluids. It has been suggested that the combination of absorption processes as described by polymer solution theory and adsorption processes that occur within the nanopores of coal macerals accurately predicts the selective expulsion of hydrocarbon gases while retaining larger C15+ compounds. Such processes may well occur within coals, but may not be relevant to oil-prone kerogens.
On the other hand, kerogens behave in many ways very similar to synthetic cross-linked polymers. When dealing with the swelling of such polymeric systems, the elastic restoring force of the connected polymer network also must be considered. Polymer science has developed a number of theories of varying complexity to explain this behavior. Conceptually, these theories predict that a highly cross-linked polymer cannot uncoil very much by solvent swelling before the elastic restoring force overcomes the entropy of mixing. As one example, the Flory-Rehner theory of rubber elasticity is comparatively simple and relates the degree of swelling to the average molecular weight between cross-links.
While the composition of the expelled petroleum fluid modeled at 50% fractional conversion is similar to that seen in produced oils, the presence of polar-rich fluids at higher levels of thermal maturation is not consistent with natural occurrences. This is not a flaw in the expulsion model. Rather, it indicates that the composition of the primary products are not fixed, as suggested by open-system laboratory experiments, but changes within the kerogen matrix as a substantial proportion of the evolved polar compounds undergo secondary cracking reactions. By incorporating reaction pathways for the thermal decomposition of polar compounds within a multi-component hydrocarbon generation model, the composition of the non-expelled petroleum fluid can be calculated under geologic heating conditions.
Unfortunately, a complete solution of the expulsion model based on the extended Flory-Rehner and Regular Solution Theory framework is computationally intense and impractical for use within another program that models petroleum generation and secondary cracking. An improved method of modeling basin performance, including predicting petroleum production, is desirable.